Wall unit plugged into split plug on dbl 15 breaker. After running 1-2 hrs. breaker trips. Wire warm to hot and plug receptacle warm. Is problem probably the air conditioner or perhaps plugin or breaker. Jim

If the breaker is tripping before the wire gets too hot, it is doing exactly what it is meant to do. My guess is that part of the coils in the motor are shorted, leading to drawing excess current. Could be dry bearings too. Check the nameplate on the air conditioner. If it draws more than 12 amps, and never ran right on that circuit, it is possible it just needs a heavier feed. Otherwise the motor probably will burn out completely soon.